1. Field of the Invention
The present invention relates to a power circuit for a display driver of a display device, particularly to a driving circuit of a liquid-crystal device. More particularly, the present invention relates to a display device for performing multiplex driving, and still more particularly relates to a power supplying circuit for driving a liquid-crystal device used for a liquid-crystal-driving circuit.
2. Related Background Art
FIGS. 8 and 9 show conventional liquid-crystal-driving-power supplying methods for multiplex driving a liquid-crystal device.
FIG. 8 is an illustration showing the resistance-dividing type power supplying method for driving a liquid crystal at a ⅓ bias. Reference numeral 101 denotes a battery serving as a power source, reference numeral 102 denotes a regulator connected to the battery 101 for keeping a voltage supplied from the battery 101 constant, and reference numerals 103, 104, and 105 denote resistors connected in series and having equal resistance values. One end of the resistor 103 is connected to the output of the regulator 102 and one end of the resistor 105 is connected to the negative side of the battery 101 and the ground side of the regulator 102 so as to divide an output voltage of the regulator 102. In this case, when assuming the output voltage of the regulator 102 as 4.5 V, a voltage of 3.0 V can be obtained at the connection point between the resistors 103 and 104 and a voltage of 1.5 V can be obtained at the connection point between the resistors 104 and 105.
Reference numeral 106 denotes an LCD driver (LCDDR) for driving an LCD, which receives a voltage from the battery 101 and an output of the regulator 102 and voltages divided by the resistors 103, 104, and 105 as LCD driving voltages.
Reference numeral 107 denotes a liquid-crystal panel connected to the LCD driver, which is turned on/off by the LCD driver 106.
FIG. 9 is an illustration showing a charge-pump type device for driving a liquid crystal at a ⅓ bias, in which reference numeral 108 denotes a battery serving as a power source, reference numeral 109 denotes a regulator connected to the battery 108 to keep a voltage supplied from the battery 108 constant, and reference numeral 110 denotes a charge-pump circuit for raising the voltage kept constant by the regulator 109. The charge-pump circuit outputs an input voltage, and the twofold-raised voltage and threefold-raised voltage of the input voltage. In this case, when assuming an output voltage of the regulator 109 as 1.5 V, 3.0 V is obtained as a twofold-raised voltage and 4.5 V is obtained as a threefold-raised voltage.
Reference numeral 111 denotes an LCD driver (LCDDR) for driving an LCD, which receives power from the battery 101 and a liquid-crystal-driving voltage from the charge pump 110. Reference numeral 112 denotes a liquid crystal panel connected to the LCD driver, which is turned on/off by the LCD driver 106.
However, since in the case of the use of the voltage divider using resistance shown in FIG. 8 in the above conventional examples, divided resistors are used to generate three types of voltages serving as LCD driver voltage sources, it is necessary to supply tens of microamperes or more to the current for the resistors of the voltage divider. Therefore, a mobile unit for always displaying data by using a battery as a power source has a problem that the service life of the battery is early completed because it consumes a large amount of power.
Moreover, the charge pump type device shown in FIG. 9 requires an oscillation circuit serving as a signal source for performing pumping-up and a capacitor for accumulating electric charges. Therefore, when the number of divided voltages for driving a liquid crystal increases, it is necessary to increase the number of capacitors by the increased number of divided voltages. Therefore, when the number of divided voltages to be raised increases, a problem occurs that the voltage ratio between the lowest voltage and the highest voltage increases and higher-side voltages do not become accurate integral multiples due to a switching loss and the like. Moreover, to supply stable power by using a charge-pump system, a circuit configuration becomes complex because it is necessary to stabilize power by using voltage stabilizer such as a series regulator and then boosting the power by a charge pump.
Furthermore, though some of the liquid-crystal drivers respectively having a built-in a charge pump circuit are marketed, most liquid-crystal drivers do not have a charge pump in general. Therefore, because charge-pump ICs for supplying LCD driver voltages are hardly marketed, it is actually difficult to constitute an LCD driver of a small current consumption by using a consumer IC.